Around the horn is a warm up drill that enforces hustle and accurate throwing.  

The players line up as shown and the rest of the players line up behind the shortstop.  

On the diagram, the H designates a hitter or coach.

The drill starts with the coach (H) hitting the ball to the first shortstop.  

The shortstop fields the ball and throws to first.  Shortstop then runs to first base while first baseman is throwing to 2nd base.  1st base then follows his/her throw to second and keeps going around the horn in the same manner.

Catcher tosses ball to coach and then runs to the end of the SS line.  The drill continues with the same rotation.  Once the kids get good at it, you can introduce a second ball.

This is how the original players positions will change after one complete rotation.

The drill continues.

Note:  Make sure the catcher runs outside the field of play around 3rd base, so they are not in the way of the next ground ball.